1. Field of the Invention
The present invention relates to a protection circuit for a field effect transistor. More particularly, the present invention relates to an inexpensive protection circuit for a field effect transistor capable of protecting the field effect transistor at a low electric power.
2. Description of Background Information
The field effect transistor (hereinafter referred to as FET) which has a high impedance and consumes a very small amount of electric power, is widely used as a switching element for a computer and an electric power control element. A maximum voltage (VSD) which can be applied between a drain and a source is set to the FET, depending on the kind and specification thereof.
If an applied voltage between the drain and the source exceeds a set value, even instantaneously, the FET will be destroyed. Therefore an overvoltage protection circuit is used for the FET, to which a voltage higher than the set voltage may be applied. Examples of conventional overvoltage protection circuits of this kind having a Zener diode are described below with reference to FIGS. 2 and 3.
With reference to FIG. 2, reference numeral 1 denotes a battery, and reference numeral 2 denotes a load, such as, for example, a lamp mounted on a vehicle. Reference numeral 3 denotes a Zener diode, and reference numeral 4 denotes an FET. Reference numeral 5 denotes a protection resistance of a gate G of the FET 4. Reference numeral 6 denotes a level-fixing resistance for fixing the gate voltage of the FET to a certain voltage when an output of a microcomputer is off. The Zener diode 3 is provided in a reverse direction between a drain D of the FET 4 and a ground.
In this circuit, when a voltage is applied to the gate G of the FET 4 from the microcomputer M, the FET is turned on and electric current flows from the drain D to a source S. Thus the lamp 2 is turned on by the voltage of the battery 1.
When an overvoltage higher than a Zener voltage of the Zener diode 3 is applied to the drain D of the FET 4, a voltage between the drain D of the FET 4 and the source S thereof is kept at the Zener voltage by the Zener effect. Thereby the FET 4 is protected.
With reference to FIG. 3, a Zener diode 7 is provided in a reverse direction between a drain D of an FET 4 and a gate G thereof. When an overvoltage higher than a Zener voltage of the Zener diode 7 is applied to the drain D of the FET 4, a voltage is applied to the gate G of the FET 4 through the Zener diode 7 due to the Zener effect. As a result, the FET 4 is turned on. Thus the FET 4 is protected by dropping the voltage between the drain D and the source S to a voltage lower than the maximum voltage (VSD).
However, in the protection circuit of FIG. 2, because the energy of the overvoltage of the Zener diode 3 is consumed, a high energy is applied to the Zener diode 3. Thus the Zener diode 3 is required to have a high wattage. Therefore it is necessary to use a large Zener diode, which requires a large space for mounting. In this case, the cost of the protection circuit is high because the large Zener diode is expensive.
In the protection circuit of FIG. 3, because the Zener diode 7 is provided in a reverse direction between the drain D of the FET 4 and the gate G thereof, leak current of the Zener diode 7 is high. Thus even when the overvoltage is not applied (normal state) to the drain D of the FET 4, the FET 4 may be turned on. Thus there is a possibility that the lamp 2 may be turned on erroneously.
According to one aspect of the present invention, it is an object of the present invention to provide a protection circuit of a field effect transistor which can be constructed at a low cost, protect an FET securely, and is prevented from being affected by leak current or the like in its normal operation.
To achieve the objects, according to an aspect of the present invention, there is provided a protection circuit of a field effect transistor connected to a load and driving the load by a voltage applied to a gate thereof and protected from an overvoltage inputted from the load. A Zener diode and at least one transistor are connected in series with each other in a reverse direction between a drain of the field effect transistor and the gate thereof; and the field effect transistor is turned on when an input of the overvoltage is transmitted in the order of the Zener diode and the transistors.
According to the present invention, if an overvoltage applied to the drain of the FET from the load is higher than the Zener voltage of the Zener diode, Zener current flows through the Zener due to the Zener effect. As a result, the transistor is turned on, and the FET is turned on. Thus the overvoltage applied between the drain and the source is restrained to a low voltage.
Therefore, it is unnecessary to consider leak current of the Zener diode and it is possible to prevent the leak current from adversely affecting a normal operation of the FET.
Further, because it is possible to select a Zener diode having a Zener voltage lower than the maximum allowable voltage of the FET, the space necessary to mount the Zener diode can be reduced by using a small Zener diode. Thus the protection circuit can be manufactured at a low cost.
To achieve the objects, according to a further aspect of the present invention, a current-limiting resistance for limiting electric current flowing through the Zener diode is connected in series with the Zener diode.
In this case, it is possible to reduce the wattage of the Zener diode by limiting electric current flowing therethrough. Thus it is possible to use an inexpensive, small Zener diode having a low wattage.
According to a further aspect of the present invention, a protection circuit for a field effect transistor connected to a load and driving the load by a voltage applied to a gate thereof and protected from an overvoltage from the load is provided including a Zener diode and at least one transistor connected in series in a reverse direction between a drain of the field effect transistor and the gate thereof, and wherein the field effect transistor is turned on when an input of the overvoltage is transmitted to the Zener diode and the at least one transistor. Furthermore, the field effect transistor may be turned on when an input of the overvoltage is transmitted to the Zener diode and the at least one transistor in the order of the Zener diode and then to the at least one transistor. The Zener diode and the at least one transistor connected in series in a reverse direction between the drain and the gate of the field effect transistor prevents leak current of the Zener diode from adversely affecting operation of the field effect transistor.
In a further aspect of the present invention, the at least one transistor connected in series in a reverse direction with the Zener diode between the drain and the gate of the field effect transistor includes one transistor. Furthermore, the at least one transistor connected in series in a reverse direction with the Zener diode between the drain and the gate of the field effect transistor may include two transistors or three transistors.
In a further aspect of the present invention, a protection circuit for a field effect transistor is provided with a low wattage transistor. The protection circuit for a field effect transistor may include a small transistor. Furthermore, the present invention may include a Zener diode which requires a small amount of mounting space.